1. Field of the Invention
The present invention relates to a technique of a detecting device for detecting light to be detected by rejecting light having an unnecessary wavelength, and of an optical apparatus including the detecting device.
2. Description of the Related Art
A fluorescence observation is widely known as one of sample observing methods. A fluorescence observation is an observation method for observing a sample by illuminating excitation light on the sample dyed with a fluorescent substance, and by detecting fluorescence light generated from the fluorescent substance with a photodetector.
Normally, fluorescence light generated from a fluorescent substance is much weaker than excitation light needed to excite the fluorescent substance. Therefore, the following two points are vital in a fluorescence observation. [a] Detecting as much fluorescence light as possible with a photodetector. [b] Sufficiently rejecting excitation light before the excitation light reflected on a sample or an optical element on an optical path is incident to a photodetector.
A conventional fluorescence microscope used for a fluorescence observation separates fluorescence light and excitation light by using an optical filter such as an absorption filter or the like in addition to a dichroic mirror. Normally, a fluorescence wavelength and an excitation wavelength are relatively close. Therefore, to sufficiently attenuate excitation light while efficiently transmitting fluorescence light, a transmittance characteristic having a steep rise or fall for a wavelength (simply referred to as a steep transmittance characteristic hereinafter) is demanded for an optical filter. However, it is difficult to design an optical filter having such a steep characteristic, and a trade-off exists between a transmission of fluorescence light and rejection of excitation light. This trade-off becomes more noticeable if an optical filter is designed to attenuate a plurality of excitation light beams having different excitation wavelengths.
In light of the above described circumstances, a technique of rejecting excitation light by using a diffraction grating and a light-shielding plate as a replacement for an optical filter is disclosed by Japanese Laid-open Patent Publication No. 2006-125970.
According to Japanese Laid-open Patent Publication No. 2006-125970, incident light is refracted in different directions for respective wavelengths by using a diffraction grating. Consequently, light emitted from the diffraction grating results in dispersed light where component light beams having different wavelengths are arranged in order of the wavelengths. By arranging a light-shielding plate in a region through which component light having an excitation wavelength passes, the excitation light can be shielded. Since the light-shielding plate is configured to be movable, it can also cope with a change in a refractive grating or a change in an excitation wavelength.
To reject excitation light without shielding fluorescence light by using the technique disclosed by Japanese Laid-open Patent Publication No. 2006-125970, it is preferable to arrange the light-shielding plate only in a region through which component light having an excitation wavelength passes. It is also preferable to reduce the size of the light-shielding plate to a minimum needed to shield the component light having the excitation wavelength. The reason is to prevent component light (fluorescence light) having a wavelength different from the excitation wavelength from being shielded by the light-shielding plate.